1. Field of the Invention
The present invention relates to a decision feedback equalizer of communication systems. More specifically, the present invention relates to a soft-threshold-based multi-layer decision feedback equalizer and decision method of a receiver of a communication system, such as CD driver, hard disk driver and ether network.
2. Description of the Related Art
In communication systems, the transmitted signal will be dispersed by the channel to interfere the neighboring signals. This impairment is called Inter-Symbol Interference (ISI).
To solve the problem, a Decision Feedback Equalizer (DFE) is employed. The block diagram of the conventional decision feedback equalizer is as shown in FIG. 1. The conventional feedback equalizer includes a feed forward filter 11, a feed backward filter 12, an adder 13 and a decision device 14. rn is the output of the equalizer and can be expressed as
            r      n        =                            ∑                      m            =            0                                              N              b                        -            1                          ⁢                              b            m                    ×                      y                          n              -              m                                          -                        ∑                      m            =            1                                N            a                          ⁢                              a            m                    ×                                    x              ^                                      n              -              m                                            ,where am denotes the m-th tap-weight of the feed backward filter,
bk denotes the (k+1)-th tap-weight of the feed forward filter,
Na is the number of taps in the feed backward filter,
Nb is the number of taps in the feed forward filter.
In the conventional decision feedback equalizer, the decision device is a slicer. The slicer maps the transmitted signal into the closest output of the decision feedback equalizer. Take 2-PAM {+1, −1} for example, when the output of the decision feedback equalizer is larger than zero, the slicer outputs 1; otherwise, it outputs −1. However, the slicer can be exactly operated only the conventional decision feedback equalizer is in the circumstance with higher SNR (Signal-to-Noise Ratio). If the SNR is not high enough, the slicer would make the decision device to make a decision error. And, this error will be feed back to the feed backward filter to cause error propagation. The performance of the system is reduced.
To solve this problem, F. Zahao, G. Mathew and B. Farhang-Boroujeny disclose a decision feedback equalizer (TT-DEF) in an academic essay, entitled “Techniques for Minimizing Error Propagation in Decision Feedback Detectors for Recording channels” in IEEE Trans. Mag., Vol. 37, Issue: 1, pp. 592-602, published on January 2001. For the TT-DEF, when decision error arises at time n, it will cause a large offset at next time, n+1. By detecting the large offset, the error event at time n can be detected and recovered. The TT-DFE operates as the following equation:rn=f0xn+εnd,Type 1:rn+1=(f0−2a1)xn+1+εn+1d, for xn+1=xn,Type 2:rn+1=(f0+2a1)xn+1+εn+1d, for xn+1=−xn,where f0 is one tap-weight of the feed forward equalizer. It approximates to 1; εnd is the noise.
As shown in FIG. 2, the TT-DFE employs a threshold test to detect two types of decision error at time n, based on rn, rn+1, xn and Xn+1 values, which is expressed as:
  Type  ⁢          ⁢  1  ⁢      :    ⁢          ⁢      {                                                                                                                              r                    n                                                                    <                                  α                  1                                            ,                                                                                                                                                  r                                          n                      +                      1                                                                                        >                                  β                  1                                            ,                                                                                                                                x                    ⋒                                                        n                    +                    1                                                  ≠                                                      x                    ^                                    n                                            ,                                          ⁢                          ⁢      or      ⁢                          ⁢      Type      ⁢                          ⁢      2      ⁢              :            ⁢                          ⁢              {                                                                                                                                            r                      n                                                                            <                                      α                    2                                                  ,                                                                                                                                                                        r                                              n                        +                        1                                                                                                  <                                      β                    2                                                  ,                                                                                                                              x                    ⋒                                                        n                    +                                                  =                                                                            x                      ^                                        n                                    .                                                                        where α1,α2, β1 and β2 are design parameters.